This invention relates to the field of air pollution control, and is particularly directed to an improved venturi scrubbing system for removing contaminants from a gaseous effluent stream, such as the output of an incinerator.
Over the past several decades the control of air pollution has become a priority concern of society. The United States, and other countries, have developed highly elaborate regulatory programs aimed at requiring factories, and other major sources of air pollution, to install the best available control technology (BACT) for removing contaminants from gaseous effluent streams released into the atmosphere. The standards for air pollution control are becoming increasingly stringent, so that there is a constant demand for ever more effective pollution control technologies. In addition, the operating costs of running pollution control equipment can be substantial, and so there is also a constant demand for more efficient technologies.
Concerns about pollution control are directed to more than air pollution, and removing contaminants from one medium frequently results in their introduction into another. For example, the treatment of municipal wastewater under the Clean Water Act has resulted in an enormous increase in the amount of sewage sludge that must be disposed of. Many communities lack adequate disposal sites to discard sludge that is generated by their municipal wastewater treatment plants in landfills, and are turning to incineration as an alternative method of disposal. Incineration of sludge, or other waste products, while greatly reducing the volume of material that must be disposed of on land, may result in the release of contaminants in the sludge into the atmosphere. In this regard, it is noted that the sludge generated by many municipalities is contaminated by highly toxic heavy metals and organic compounds, as well as acidic compounds such as chlorides and sulfates. The release of such compounds into the atmosphere is highly regulated, and sludge incineration systems are required to use BACT for controlling the release of contaminants into the atmosphere.
One well-known type of device for removing contaminants from a gaseous effluent stream is a venturi scrubber. Venturi scrubbers are generally recognized as having the highest fine particle collection efficiency of available scrubbing devices. As the name implies, in a venturi scrubber the effluent gas is forced or drawn through a venturi tube having a narrow xe2x80x9cthroatxe2x80x9d portion. As the gas moves through the throat it is accelerated to a high velocity. Droplets of a scrubbing or cleansing liquid, typically water, are created in the venturi, usually in the vicinity of the throat, and enter the gas flow. The droplets used are generally many orders of magnitude larger than the contaminant particles to be collected and, as a consequence, accelerate at a different rate through the venturi. The differential acceleration causes interactions between the droplets and the contaminant particles, such that the contaminant particles are collected by the droplets. The collection mechanisms involve, primarily, collisions between the particles and the droplets and diffusion of particles to the surface of the droplets. In either case, the particles are captured by the droplets. Depending on the size of the contaminant particles, one or the other of these mechanisms may predominate, with diffusion being the predominant collection mechanism for very small particles, and collision or interception being the predominant mechanism for larger particles. A venturi scrubber can also be efficient at collecting highly soluble gaseous compounds by diffusion. A detailed description of these scrubbing mechanisms is discussed in Chapter 9 of Air Pollution Control Theory, M. Crawford, (McGraw-Hill 1976).
After the particulate contaminants are collected by the scrubbing droplets, the droplets are then removed from the effluent stream which is thereby cleansed. Removal of the droplets may be accomplished by a number of known means, which typically rely on the fact that the scrubbing liquid droplets are relatively large and, due to inertia, cannot change direction rapidly. Thus, to remove the droplets, the gas flow may be directed toward a surface such as an impingement plate. While the gas moves around the surface, the inertia of the relatively large droplets causes them to strike the surface where they are captured. Likewise, the droplets may be captured by a circular flow, as in a cyclonic separator, where the relatively large droplets collide with the wall of the separator due to centrifugal force.
Most venturi scrubbers in use today are xe2x80x9cself-atomizing,xe2x80x9d i.e., the droplets are formed by allowing a liquid to flow into the throat of the venturi where it is atomized by the gas flow. When operated at their design conditions, these systems are not able to produce droplets of very small mass median diameter, typically 500-2000 microns.
The primary methods heretofore utilized in improving the collection efficiency of a venturi scrubber have been to decrease the size of the throat or to increase the overall rate at which gas flows through the system. Both of these methods increase the differential velocities between the contaminant particles and liquid droplets as they pass through the throat of the venturi. This causes more interactions between particles and droplets to occur, thereby improving contaminant removal. However, increasing the collection efficiency in this manner comes at a cost of significantly higher energy input into the system, thereby resulting in higher operating costs. The extra energy is expended due either to the increased overall flow resistance attributable to the reduced throat diameter, or to the increased overall flow rate through the venturi. In either case, the pressure drop across the venturi is increased and greater pumping capacity is required. Most prior art efforts to increase the fine particle collection efficiency of a venturi scrubber have involved substantial increased energy input into the system.
Of particular concern to those in the field of air pollution control is the collection of xe2x80x9coptically activexe2x80x9d particles. As used herein, the term xe2x80x9coptically active particlesxe2x80x9d should be understood to mean particles having a diameter in the range of approximately 0.1 to 1.0 microns. In an effort to control these particles, the EPA has recently set xe2x80x9cPM2.5 standardsxe2x80x9d for the emissions of particles less of than 2.5 microns. These particles are difficult to collect in conventional venturi scrubbers due to their small size. Nonetheless, particles in this size range often comprise toxic material the release of which is not permitted. Due to the relatively large surface area of optically active particles, they absorb a disproportionate amount of heavy metal contamination. As their name implies, optically active particles interact with light. Even if they do not contain toxic components, the emission of optically active particles is highly visible and undesirable from an aesthetic point of view.
As noted above, municipal sewage sludge often contains significant amounts of toxic heavy metal and organic materials. Heretofore, scrubbers have not been efficient in removing these materials from the gaseous effluent of incinerated sludge. Municipal sewage sludge incineration typically requires the use of high temperatures (i.e., between 900xc2x0-1,600xc2x0 F.). At these elevated temperatures, the organic materials are vaporized and are, thus, not susceptible to efficient scrubbing. One approach to this problem has been to use an afterburner on the effluent stream, whereby the organic vapors are combusted and, thereby, transformed into non-toxic compounds, primarily water vapor and carbon dioxide. However, incomplete combustion of the organics can result in the production of carbon monoxide, soot, and/or gaseous hydrocarbons. If soot (i.e., fine particles of carbon) is produced, other compounds, such as those containing heavy metals, can be adsorbed on the surface of the carbon particles. Any particles that are formed in this way are likely to be difficult to collect due to their small diameter. And, as noted above, very small particles are efficient collectors of volatile heavy metals.
In co-assigned U.S. Pat. No. 5,279,646, (hereafter the ""646 patent) by the inventor hereof (the disclosure of which is incorporated by reference), it is taught to optimize the size of the scrubbing droplets to promote the maximum collection efficiency for optically active contaminant particles. This patent describes the fact that there is a point at which a further decrease in the size of the droplets of the scrubbing liquid begins to become detrimental. The ""646 patent teaches a method and apparatus for creating optimized droplets which are introduced into the effluent gas flow upstream of the venturi throat. The ""646 patent further teaches the use of a two-fluid nozzle to create droplets of a scrubbing or cleansing liquid. The inventive apparatus and method of the ""646 patent have proven to be quite successful when using the preferred two-fluid nozzle.
The ""646 patent notes that certain hydraulic (i.e., liquid only) nozzles are capable of producing droplets in the optimal range and could be used in practicing the invention described in the patent. However, as a practical matter, it has proven difficult to achieve all of the objectives of the ""646 patent when using a hydraulic nozzle.
As noted, the trend in pollution control has been towards increased stringency, such that many existing facilities face the need to upgrade or retrofit their existing pollution control equipment to achieve better results. In addition, facility owners/operators are often interested in upgrading or retrofitting existing pollution control equipment to realize the benefit of lower operational costs from improved efficiency.
In many situations, when retrofitting or upgrading an air pollution control system it is difficult due to space or power considerations to provide the pressurized air needed to operate the two-fluid nozzles described in the ""646 patent. Therefore, in such situations, it is difficult to realize the benefits described in the patent.
What is desired is an apparatus and method that permits the efficient and economical scrubbing of fine particles from a gas flow using a cleansing liquid in a venturi scrubber. Specific needs include reduced scrubbing liquid pumping requirements, lower pressure drop across the venturi, improved scrubber performance, and better control of the pressure drop across the venturi scrubber.
The present invention generally comprises an apparatus and method to create a spray of fine droplets composed of a scrubbing liquid for scrubbing particulates from a contaminated gas. The scrubber includes a venturi having an inlet for receiving a contaminated gas, a throat and an outlet.
It is one aspect of the present invention to provide a venturi scrubber having a first nozzle to introduce fine droplets of a first cleansing liquid into the gas flow, and positioned upstream of the throat; and a second nozzle to introduce fine droplets of a second cleansing liquid into the gas flow, and positioned within said throat and oriented to introduce droplets with a component of velocity which is counter to the direction of gas flow through the venturi.
It is another aspect of the present invention to provide a means for introducing fine droplets of a first cleansing liquid into said flow of gas through the venturi and upstream of the throat; and means for introducing fine droplets of a second cleansing liquid into the throat in a direction counter to the direction of gas flow through the venturi.
It is yet another aspect of the present invention to provide a venturi scrubber including a nozzle to introduce fine droplets of a cleansing liquid into said gas flow, where said droplets are introduced into said throat with a component of velocity which is counter to the direction of gas flow through said venturi, and where the flow of said cleansing liquid is selected so that the pressure drop across said venturi scrubber is approximately equal to a specified pressure drop.
It is an aspect of the present invention to provide a method of cleansing a gas flow using a venturi scrubber at a prescribed pressure drop across a venturi. The method includes the steps of injecting fine droplets of cleansing liquid counterflow to the gas flow and into the throat, and where the flow of cleansing liquid has a valve to adjust the flow; and adjusting the valve to maintain said prescribed pressure drop across the venturi.
It is yet another aspect of the present invention to provide a method of providing a retrofit for a pre-existing venturi scrubber in an air pollution control system, where the pre-existing venturi has a prescribed pressure drop. This method includes the steps of installing components within said pre-existing venturi including a nozzle to introduce fine droplets of a cleansing liquid into the throat of the venturi with a component of velocity which is counter to the direction of gas flow through the venturi; and selecting the flow rate of cleansing liquid so that the pressure drop across said venturi is approximately equal to the prescribed pressure drop.
A further understanding of the invention can be had from the detailed discussion of specific embodiments below. For purposes of clarity, this discussion refers to devices, methods, and concepts in terms of specific examples. However, the method of the present invention may operate with a wide variety of types of devices. It is therefore intended that the invention not be limited by the discussion of specific embodiments.